1. Field of the Invention
The present invention relates to an apparatus for adjusting a valve lift (hereinafter called as a variable valve lifting, VVL, apparatus) which is used in a direct-strike type of valve driving system in which a tappet is directly stricken by a cam when an intake valve or an exhaust valve (hereinafter referred to as a valve altogether) of an internal combustion engine (hereinafter called an engine) is opened or closed, in which an amount of valve lifting (hereinafter called a valve lift amount) is adjusted by changing an axial length of the tappet.
2. Description of the Prior Art
FIG. 24 is a schematic view showing an arrangement of a general valve system of an engine. FIG. 25 is a view as seen in the direction of arrows Axe2x80x94A showing an arrangement of cams on a camshaft in the valve driving system in FIG. 24. FIG. 26 is a front view showing a profile of the cam in FIG. 25. FIGS. 27A through 27G are schematic sectional views to show a conventional valve opening movement and valve closing movement of the VVL apparatus shown in FIG. 24 in a low lift mode, and FIG. 27H is a graph continuously showing the change in the valve lift amount. FIGS. 28A through 28G are schematic sectional views to show the conventional valve opening movement and valve closing movement of the VVL apparatus shown in FIG. 24 in a high lift mode, and FIG. 28H is a graph continuously showing the change in the valve lift amount.
In the example given hereinbelow, only the valve driving system on the intake side is shown out of the valve driving system for each of the intake side and the exhaust side. Since the valve driving system on the exhaust side has basically the same arrangement, an explanation thereabout is omitted. In addition, explanations are made hereinbelow based on the presumption that the cylinders in the figures are disposed in a vertical direction.
In the figures, each reference numeral 1, 2, 3, 4 denotes a cylinder of a 4-cylinder engine (hereinafter simply called a cylinder). Inside each of these cylinders 1, 2, 3 and 4, there is disposed a piston 5, 6, 7, 8 which reciprocates in an axial direction of the respective cylinders. The reciprocating movements of these pistons 5, 6, 7 and 8 are converted into rotary movements by crank mechanisms 9, 10, 11 and 12 for transmission to a crankshaft 13. On an upper portion (cylinder head) of each of the cylinders 1, 2, 3 and 4, there are provided two valve seats 14, 15, 16 and 17, respectively. An intake valve 18, 19, 20, 21 is disposed in each of the valve seats 14, 15, 16 and 17. The intake valves 18, 19, 20 and 21 are arranged to be subject to rotational driving of intake-side cams 26, 27, 28 and 29, respectively, through an apparatus for adjusting a valve lift (hereinafter called a variable valve lifting, VVL, apparatus) 22, 23, 24, 25. Intake-side cams 26, 27, 28 and 29 are disposed on an intake-side camshaft 30. The intake-side camshaft 30 is rotatable in a direction as shown in FIG. 25 by an arrow B by a rotational driving force to be transmitted by a drive transmission member 32 such as a pulley 31, a timing belt, or the like, as well as a pulley 33.
Here, since the intake-side cams 26, 27, 28 and 29 have all the similar construction, an explanation will now be made about the intake-side cam 26 as a typical example. The intake-side cam 26 is made up, as shown in FIG. 26, of a base circle section 26a of a true circle in cross section, a lift curve section 26b which is raised from this base circle section 26a, and ramp sections 26c and 26d which smoothly connect the base circle section 26a and the lift cure section 26b together. This arrangement is the same as that of the remaining intake-side cams 27, 28 and 29.
As shown in FIG. 25, the lift curve section 27b of the intake-side cam 27 and the lift curve section 28b of the intake-side cam 28 are disposed at an offset of about xc2x190xc2x0 on the periphery of the intake-side camshaft 30 relative to the lift curve section 26b of the intake-side cam 26. The lift curve section 29b of the remaining intake-side cam 29 is disposed at an offset of about 180xc2x0 on the periphery of the intake-side camshaft 30 relative to the lift curve section 26b of the intake-side cam 26.
Since the above VVL apparatuses 22, 23, 24 and 25 have all the similar constructions, an explanation will now be made about the VVL apparatus 22 as a typical example. As shown in FIGS. 27 and 28, the VVL apparatus 22 is roughly made up of: a tappet casing 34 having on an upper portion thereof a cam contact portion 34a which comes into contact or abutment with a cam surface of the intake-side cam 26; and a hydraulic cylinder (not illustrated) which is disposed inside the tappet casing 34 and which switches between a high lift mode in which a tappet axial length is extended and a low lift mode in which the tappet axial length is contracted. This arrangement is disclosed in German Patent Publication DT1958627. The lower portion of this VVL apparatus 22 is in contact or abutment with an upper portion of a valve stem 35. On a lower portion of this valve stem 35 there is provided the above-described intake valve 18. Between this valve stem 35 and the cylinder 1 there is disposed a valve spring 36 which urges the valve stem 35 axially upward to urge the intake valve 18 against the valve seat 14, whereby the intake valve 18 is brought into a closed state.
An explanation will now be made about the operation of the VVL apparatus 22.
First, right after starting the engine, since the hydraulic pressure to be supplied from an oil pump (not illustrated) to the VVL apparatus 22 is not high enough, the hydraulic cylinder (not illustrated) inside the VVL apparatus 22 has not extended yet, whereby a setting is made to the low lift mode. In the low lift mode, the intake-side cam 26 rotates in the direction of an arrow B. As shown in FIGS. 27A and 27B, the cam contact surface 34a of the tappet casing 34 comes into contact with the intake-side cam 26 from the base circle section 26a through the ramp section 26c toward the lift curve section 26b. However, since the axially downward displacement of the cam contact portion 34a is still small, the tappet casing 34 and the valve stem 35 do not move axially downward yet. As a result of further rotation of the intake-side cam 26 and, as shown in FIGS. 27C through 27E, when the cam contact portion 34a of the tappet casing 34 proceeds to come into contact with the ramp section 26c toward the central portion (nose) of the lift curve section 26b, the axially downward displacement of the cam contact portion 34a increases. Therefore, the tappet casing 34 and the valve stem 35 are pushed downward against the urging force of a helical (or coiled) spring 36. As a consequence, the intake valve 18 is also pushed axially downward relative to the valve seat 14 to attain the low lift state. The valve lift amount at this time gradually increases, as shown in FIG. 27H, from the stage corresponding to FIG. 27C (valve open) and becomes maximum at the stage corresponding to FIG. 27D. As a result of further rotation of the intake-side cam 26 and, as shown in FIG. 27E, once the cam contact portion 34a of the tappet casing 34 comes into contact, through the central portion of the lift curve section 26b, with the ramp section 26d, the axially downward displacement of the cam contact portion 34a moves in the direction of decreasing. Therefore, the tappet casing 34 and the valve stem 35 are lifted by the urging force of the helical spring 36 while following the cam profile of the intake-side cam 26. As a result, the intake valve 18 is also forced against the valve seat 14, whereby the lifted state is finished (valve closed). The valve lift amount at this time starts to decrease, as shown in FIG. 27H, from the stage corresponding to FIG. 27D and becomes zero at the stage corresponding to FIG. 27E (valve closed). The finishing of the lifted state continues also during the process, as shown in FIGS. 27F and 27G, in which the cam contact portion 34a of the tappet casing 34 contacts the ramp section 26d toward the base circle section 26a. 
In this kind of low lift mode, since the hydraulic pressure inside the hydraulic cylinder is low at the time of low-speed rotation of the engine, the cylinder length corresponding to the tappet axial length is shortened to thereby set the valve lift amount to the low lift. As a result, the flow speed of air-fuel mixture can be increased to improve the combustion efficiency.
At the time of ordinary driving of the engine, the hydraulic pressure to be supplied from the oil pump (not illustrated) to the VVL apparatus 22 has already been sufficiently high. The hydraulic cylinder (not illustrated) inside the VVL apparatus 22 is thus extended to thereby set the mode to a high lift mode. In the high lift mode, the intake-side cam 26 rotates in the direction of the arrow B. As shown in FIG. 28A, the cam contact portion 34a of the tappet casing 34 proceeds to contact the intake-side cam 26 from the base circle section 26a toward the ramp section 26c. However, since the axially downward displacement of the cam contact portion 34a is still small, the tappet casing 34 and the valve stem 35 do not move axially downward. As a result of further rotation of the intake-side cam 26 and, as shown in FIGS. 28B through 28D, when the cam contact portion 34a of the tappet casing 34 contacts the intake-side cam 26 from the base circle section 26a through the ramp section 26c toward the lift curve section 26b, the axially downward displacement of the cam contact portion 34a increases. Therefore, the tappet casing 34 and the valve stem 35 are gradually pushed axially downward against the urging force of the helical spring 36. As a result, the intake valve 18 is also pushed axially downward relative to the valve seat 14 to thereby attain a high lift state. The valve lift amount at this time gradually increases, as shown in FIG. 28H, from the stage corresponding to FIG. 28A (valve open) and attains a maximum value at the stage corresponding to FIG. 28D. As a result of further rotation of the intake-side cam 26 and, as shown in FIGS. 28E through 28G, when the cam contact portion 34a of the tappet casing 34 proceeds to contact the central portion of the lift curve section 26b toward the ramp section 26d, the axially downward displacement of the cam contact portion 34a moves in the direction of decreasing. Therefore, the tappet casing 34 and the valve stem 35 are lifted by the urging force of the helical spring 36 while following the cam profile of the intake-side cam 26. As a consequence, the intake valve 18 is also urged against the valve seat 14 to thereby finish the lifted state (valve closed). The valve lift amount at this time starts, as shown in FIG. 28H, to decrease at the stage corresponding to FIG. 28D and becomes zero at the stage corresponding to FIG. 27G (valve closed).
In this kind of high lift mode, by taking advantage of the fact that the drain following characteristic of the hydraulic cylinder at a high-speed rotation of the engine does not catch up, the cylinder length corresponding to the tappet axial length is maintained and the valve lift amount is maintained to a high lift amount. In this manner, a lower specific fuel consumption and a higher engine output can be attained by improving the suction efficiency.
In this kind of VVL apparatus 22, in the high lift mode, due to a clearance (not illustrated) which is provided between the intake-side cam 26 and the cam contact portion 34a of the tappet casing 34 to take into account the thermal expansion or the like, the intake-side cam 26 and the tappet casing 34 come into collision with each other as a result of the rotation of the intake-side cam 26 having the high lift profile at the time of the following: i.e., at the time when, as shown in FIG. 28A, the cam contact portion 34a of the tappet casing 34 comes into contact with the intake-side cam 26 after going through the base circle section 26a and the ramp section 26c toward the lift curve section 26b; and at the time when s shown in FIG. 28G, the contact portion 34a of the tappet casing 34 comes into contact with the suction-side cam 26 after going through the lift curve section 26b and the ramp section 26d toward the base circle section 26a. It is, however, possible to absorb and restrict the collision noises at the above-described ramp section 26c or 26d. 
In the above-described VVL apparatus 22, however, the intake-side cam 26 having the high lift profile is used also in the low lift mode. Therefore, while the collision noises attributable to the above-described clearance (not illustrated) can be restricted or kept under control, it is difficult to effectively restrict such collision noises as described hereinbelow. Namely, in the low lift mode, the axial length of the tappet casing 34 is reduced. Therefore, the time when the ramp sections 26c and 26d of the intake-side cam 26 come into contact with the cam contact portion 34a of the tappet casing 34 falls, as shown in FIGS. 27B and 27F, under such a period of reduced axial length of the tappet as will not substantially contribute to the opening and closing of the intake valve 18. Therefore, the ramp sections 26c and 26d cannot serve the function of smoothly connecting the base circle section 26a and the lift curve section 26b together. As a result, there was a problem in that the following collision noises cannot be effectively restricted. The noises in question are: collision noises which occur, at the beginning of valve lifting, between the tappet and the valve stem as a result of rapid lifting of the intake valve 18 at the time of contact of the ramp section 26c, right after the contact thereof, with the former part of the lift curve section 26b; and collision noises which occur, at the end of valve lifting, between the intake valve 18 and the valve seat as a result of rapid forcing of the intake valve against the valve seat at the time of contact of the ramp section 26d, right before the contact thereof, with the latter half of the lift curve section 26b. 
In addition, in the above-described VVL apparatus 22, there is employed no construction of locking the extension and contraction of the hydraulic cylinder at a specific point. Therefore, the valve lift amount (i.e., accuracy) depends on the amount of oil leak from the hydraulic cylinder, the rotational frequency of the engine (i.e., the speed of forcing the piston into the cylinder), or the like. As a result, it is considered difficult to set the valve lift amount to a specific value.
As a possible solution to this problem, Unexamined International Patent Publication (KOHYO KOHO) No. 507242/1998 and Japanese Published Unexamined Patent Application (KOKAI KOHO) No. 141030/1998 disclose VVL apparatuses in which a specific valve lift amount can be set. These VVL apparatuses are each substantially made up of: a plurality of cams provided on a camshaft rotatably driven by a crankshaft of an engine; an inner tappet which reciprocates in an axial direction of a valve stem to follow a cam profile of a rotary cam, out of a plurality of cams, of a low-lift cam which contributes to the opening and closing of the valve in a low rotational speed region; an outer tappet which is provided on an outside of the inner tappet and which reciprocates in the axial direction of the valve stem to follow a cam profile of a rotary cam, out of the plurality of cams, of a high lift cam which contributes to the opening and closing of the valve in a high rotational speed region; and a moving member which is disposed inside the inner tappet so as to be movable in a radial direction of the inner tappet. This moving member moves in the following manner. Namely, in a high lift mode, it moves radially outward by the hydraulic pressure supplied to the central portion of the inner tappet so as to be engaged with a recessed portion on an inner circumference of the outer tappet, whereby both the tappets are integrated together. In a low lift mode, on the other hand, it is returned radially inward of the inner tappet by an urging means such as a spring or the like in a state of low hydraulic pressure so that the moving member is disengaged from the recessed portion of the outer tappet, whereby both the tappets are separated from each other.
In this kind of VVL apparatus, two kinds of cams, i.e., a high lift cam and a low lift cam, are disposed on a single camshaft in order to open and close a single valve. Therefore, this apparatus can restrict the generation of peculiar collision noises which are generated in the arrangement utilizing a single high lift cam in the low lift mode as represented by the above-described German Patent Publication DT19658627.
In the conventional VVL apparatus, however, it is necessary to provide two kinds of cams, i.e., a high lift cam and a low lift cam, resulting in another problem in that the number of parts increases with a consequent high cost and increased weight.
The present invention has been made to solve the above-described problems and has an object of providing a VVL apparatus in which, even in a low lift mode, the collision noises can appropriately be absorbed to thereby secure a silence at the time of low rotational frequency of the engine and in which the number of parts such as cams or the like can be reduced to attain a lower cost and smaller weight of the VVL apparatus.
In order to attain the above and other objects, the present invention is an apparatus for adjusting a valve lift, comprising: a tappet casing having a cam contact portion which comes into contact with a cam provided on a camshaft rotatably driven by a crankshaft of an internal combustion engine; lift mode switching means for selectively switching between a high lift mode in which an amount of displacement of one of an intake valve and an exhaust valve of a cylinder corresponding to the tappet casing is equal to an amount of displacement of the cam contact portion of the tappet box, and a low lift mode in which the amount of displacement of the other of the intake valve and the exhaust valve relative to the amount of displacement of said one of the intake valve and the exhaust valve decreases relative to the amount of displacement of the cam contact portion; restricting means for holding the lift mode switching means to the high lift mode; and urging means for urging in a direction in which the amount of displacement of the valve by the lift mode switching means increases in the low lift mode.
Preferably, the apparatus has the following arrangements.
Namely, the cam to contact the cam contact portion of the tappet casing has a cam profile for a high lift cam which is suitable for operating conditions of one or both of above intermediate speed and above an intermediate load of the internal combustion engine.
The lift mode switching means is set such that, under operating conditions of one or both of below intermediate speed and below intermediate load of the internal combustion engine, the amount of displacement of one of the intake valve and the exhaust valve decreases relative to an amount of axial displacement of the cam contact portion of the tappet casing.
The lift mode switching means comprises: an outer tube which is disposed inside the tappet casing; and an inner tube which comes into contact with a valve stem of one of the intake valve and the exhaust valve.
The inner tube is disposed inside the outer tube so as to be relatively slidable in an axial direction of the outer tube and be relatively rotatable in a circumferential direction of the outer tube.
The inner tube has one of a projection and a recess on a periphery thereof, and the outer tube has the other of the projection and the recess which comes into engagement with said one of the projection and the recess to thereby restrict an axial movement and a peripheral movement of the outer tube.
The projection is a pin having a substantially circular cross section, and the recess is a groove having a shape engageable with the pin.
The recess has a substantially arcuate shape which is capable of gradually converting the movement of one of the inner tube and the outer tube having the projection from the axial sliding to the peripheral rotation.
The recess in the lift mode switching means is an arcuate groove of secondary curve so constructed and arranged that, when one of the intake valve and the exhaust valve operates in a direction of reducing the amount of displacement of the valve, the closer to a position of maximum decrease in the amount of displacement of the valve, the more rapid increase in an amount of circumferential displacement of the inner tube relative to the outer tube.
The outer tube is urged by urging means to urge the inner tube in a direction of increasing the amount of displacement of one of the intake valve and the exhaust valve.
The outer tube is urged by urging means to urge the cylinder in a direction of increasing the amount of displacement of one of the intake valve and the exhaust valve.
The urging means is disposed between a head portion of the outer tube and a head portion of the inner tube.
An urging force of the urging means is set, at a point of time when the amount of displacement of one of the intake valve and the exhaust valve becomes minimum relative to the amount of displacement of the contact portion of the tappet casing, so as to become larger than an urging force of a valve spring for closing the other of the intake valve and the exhaust valve.
The lift mode switching means is constituted into an element which is contained inside the tappet casing and which is separate from the tappet casing.
The restricting means mechanically engage the outer tube and the inner tube when, under one of operating conditions of above an intermediate speed and above an intermediate load of the internal combustion engine, the amount of displacement of one of the intake valve and the exhaust valve is equal to the amount of displacement of the cam contact portion of the tappet and when a state thereof is maintained.
The restricting means has an oil passage so constructed and arranged that the outer tube and the inner tube of the lift mode switching means are restricted by a supply of hydraulic pressure and that the restriction is released by lowering in the hydraulic pressure or by stopping of supply of the hydraulic pressure.
The apparatus further comprises a mechanical urging means for urging in a direction of releasing the restriction of the lift mode switching means.
The outer tube of the lift mode switching means has a communicating hole which communicates a space formed between the outer tube and the inner tube with an atmosphere.
The tappet casing is contained inside a containing hole beside the internal combustion engine so as to be axially slidable and circumferentially rotatable.